Previous tests for life on Mars have turned up negative or inconclusive. But the red planet received a promising score of 0.3 on the new Standard Microbial Habitability index, announced in October 2009.

Researchers think that the answer is to dig deeper. The sweet spot for Mars's life may be about 4 miles (6 kilometers) underground, said Mendez, of the University of Puerto Rico at Arecibo.

"You have an envelope where the temperature and pressure are ideal for microbial communities," Mendez said.